can someone id?
It looks like a fungal disease that is producing spores. Could you post some pictures of the front sides of the leaves? It would be best if the leaves were still on the tree and you had pics of several leaves. Some diseases are easier to identify at different stages of development.
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It looked like leaf rust (in a later stage) from what I compared yours to one particular pic in this article from Cornell.
https://blogs.cornell.edu/plantpathhvl/apple-diseases/rust-diseases/
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I agree it looks like rust. I think it is probably cedar apple rust but might be one of the other rusts like quince rust. To prevent it you can spray Immunox which is available at stores that carry home garden supplies like Lowes. Some people remove nearby cedar trees with mixed results since cedar trees are the alternate host of the disease. Other people ignore the disease as it often is confined to the leaves and usually doesn’t effect the fruit. However, in the South infections can be severe, damage fruit and defoliate trees. I think in PA it’s probably not a severe problem.
The other option is to plant apple trees that are resistant to the cedar apple rust. This link lists trees that have resistance.
Here is a link to a guide to home orchard diseases that I have found helpful. It includes cedar apple rust.
https://extension.uga.edu/publications/detail.html?number=B1336
I have the same thing on one of my apple trees. And I am pretty sure that it is Cedar Apple Rust, too. Even in the earlier stages - your description of a ‘wart’ is apropos.
I didn’t treat it this year. But, for next year . . . I’m ready and armed!
thanks to all
cedar rust gets my vote